Catalyst and methods for producing multi-wall carbon nanotubes

ABSTRACT

The present invention provides a catalyst precursor and a catalyst suitable for preparing multi-wall carbon nanotubes. The resulting multi-wall carbon nanotubes have a narrow distribution as to the number of walls forming the tubes and a narrow distribution in the range of diameters for the tubes. Additionally, the present invention provides methods for producing multi-wall carbon nanotubes having narrow distributions in the number of walls and diameters. Further, the present invention provides a composition of spent catalyst carrying multi-wall nanotubes having narrow distribution ranges of walls and diameters.

CROSS RELATED

This application claims the benefit of previously filed InternationalApplication PCT/US2010/042321 filed Jul. 16, 2010, and ProvisionalApplication Serial. No. 61/226,438 filed on Jul. 17, 2009.

The present application claims priority to U.S. Provisional ApplicationSer. No. 61/226,438 filed on Jul. 17, 2009. The entirety of U.S.Provisional Application Ser. No. 61/226,438 is incorporated herein byreference.

BACKGROUND OF THE INVENTION

Carbon nanotubes are known to exist in single wall and multi-wallconfigurations. Each configuration provides certain benefits. Singlewall nanotubes are preferred for electronic applications due to the lowoccurrence of structural anomalies. However, multi-wall nanotubes aregenerally lower in cost and will provide satisfactory performance inelectronic applications if the number of walls forming the nanotubes canbe controlled. Unfortunately, current methods for producing multi-wallcarbon nanotubes lack the ability to control the resulting number ofwalls in the structure in the resulting nanotubes. As a result,currently produced multi-wall carbon nanotubes generally range indiameter from about 3 to 35 nm and comprise 3 to 40 concentric graphenelayers, i.e. walls. The layers are coaxially arranged cylinders ofcarbon atoms having an interlayer distance of about 0.37 nm. This widedistribution range in walls and external diameter size limits the valueof multi-wall nanotubes for electrical conductivity applications,thermal conductivity applications and mechanical reinforcementapplications.

In contrast, multi-wall nanotubes, having a relatively narrowdistribution range of walls and external diameters, will provideelectrical conductivity characteristics approaching those of single wallnanotubes. Additionally, multi-wall nanotubes will provide suchimprovement at a lower cost. Further, multi-wall nanotubes batcheshaving narrow distributions of wall numbers and external diameters willprovide enhanced thermal conductivity and mechanical strength whencompared to batches having wide distribution ranges.

While one might consider simply isolating a narrow distribution ofmulti-wall carbon nanotubes from the wide distribution ranges presentlymanufactured, technology does not exist for carrying out this task.Thus, the currently available multi-wall nanotubes are provided solelyin batches or lots having the undesirable wide distributions of wallsand external diameters.

As discussed in detail below, the present invention provides batches ofmulti-wall nanotubes having narrow distribution ranges of walls anddiameters. When incorporated into thermoplastics the narrow distributionrange batches provide electrical conductivity characteristics whichrival single wall nanotubes and are significantly improved overcurrently available batches of multi-wall nanotubes. The currentinvention further provides catalysts and methods for preparing batchesof multi-wall carbon nanotubes having narrow distribution ranges ofwalls and external diameters.

SUMMARY OF THE CURRENT INVENTION

In one embodiment, the present invention provides a catalyst precursorcomprising alumina (Al₂O₃), magnesium oxide (MgO) and magnesiumaluminate (MgAl₂O₄) as a catalyst support. The catalyst precursorfurther comprises metallic oxides of cobalt, iron and molybdenum. Thepreferred metallic oxides include, but are not necessarily limited to,one or more of the following mixed metal oxides: CoFe₂O₄, CoMoO₄,Co_(x)MoO₄, Fe₂(MoO₄)₃, Co_(x)Fe_(y)MoO₄; where x and y represent theatomic ratios of Co and Fe relative to Mo and x is from about 1.6 toabout 6.5 and y is from about 0.1 to about 10.5. Mixed metal oxideshaving two or more metal components are preferred, as single metaloxides produce carbon fibers and other forms of carbon.

In another embodiment, the present invention provides a method forpreparing a catalyst precursor and a catalyst. The method involvesinitially preparing a solution of mixed metallic compounds comprisingtwo or more of the following: a cobalt compound selected from the groupconsisting of cobalt acetate, cobalt nitrate; an iron compound selectedfrom the group consisting of iron acetate, iron nitrate; a molybdenumcompound selected from the group consisting of ammonium heptamolybdateand ammonium dimolybdate; and magnesium nitrate. This solution isreacted with an excess of aluminum hydroxide powder and the reactionproducts subsequently formed into a paste. Formation of the paste causesthe reaction products to agglomerate thereby yielding a particle sizedistribution between about 100 and 1400 microns. The reaction productsare subsequently dried, reduced in size and calcined to yield a catalystprecursor. The currently preferred catalyst precursor has a particlesize distribution ranging from 70 μm to 150 μm. Conversion of theprecursor to a catalyst entails placing the catalyst precursor within areaction chamber suitable for use as a fluidized bed reactor. Thecatalyst precursor is fluidized and pre-heated to the desired reactiontemperature by passing an inert gas selected from the group consistingof nitrogen, argon or helium through the reaction chamber. Uponachieving steady state conditions at the desired reaction temperature,the inert gas is replaced with a blend of ethylene and inert gas. Thecatalyst precursor converts to the desired catalyst during the firstfive minutes of contact with the blend of ethylene and inert gas. Duringthe conversion process, cobalt and iron oxides are reduced to therespective metals. Additionally, a portion of the iron oxide is reducedto iron carbide (Fe₃C) and the molybdenum oxides are reduced tomolybdenum carbide (Mo₂C).

Still further, the present invention provides a method of producingmulti-wall carbon nanotubes wherein the resulting batch of multi-wallnanotubes has a narrow distribution as to the number of walls making upthe nanotubes and a narrow distribution of external diameters for theresulting nanotubes. In the method of the current invention, thecatalyst precursor is prepared as discussed above. Following conversionof the catalyst precursor to the reduced metal catalyst, flow of theethylene/inert gas continues under the desired reaction conditions for aperiod of time sufficient to yield multi-wall carbon nanotubes. Theethylene/inert gas contains from about 10% to about 80% ethylene byvolume and flows at a rate sufficient to fluidize the bed of catalystparticles. Following a reaction period of about 10 to about 30 minutes,the flow of gas to the reaction chamber is cut off and the particlescarrying the multi-wall nanotubes are removed. About 95% to about 98% ofthe resulting carbon product carried by the spent catalyst is carbonnanotubes. From about 60% to about 90% of the resulting batch ofmulti-wall carbon nanotubes have from 3 to 6 walls and externaldiameters ranging from about 3 nm to about 7 nm. Thus, the presentinvention also provides a novel product comprising carbon nanotubeshaving 3 to 6 walls and external diameters ranging from about 3 nm toabout 7 nm.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides a tabular representation of the carbon yield and carbonnanotube diameter profile for various catalyst compositions on analumina support.

FIG. 2A provides graphical representations of the carbon nanotubediameter distribution for the catalytic composition corresponding toPXE2-282 in FIG. 1.

FIG. 2B provides graphical representations of the carbon nanotubediameter distribution for the catalytic composition corresponding toPXE2-285 in FIG. 1.

FIG. 2C provides graphical representations of the carbon nanotubediameter distribution for the catalytic composition corresponding toPXE2-288 in FIG. 1.

FIG. 2D provides graphical representations of the carbon nanotubediameter distribution for the catalytic composition corresponding toPXE2-295 in FIG. 1.

FIG. 3 is a tabular representation of the effect of reaction temperatureand gas composition on the carbon yield and selectivity to smallerdiameter tubes.

FIG. 4A depicts the carbon nanotube diameter distribution as determinedby TEM corresponding to the SMW-100 carbon nanotube product.

FIG. 4B depicts the carbon nanotube diameter distribution as determinedby TEM corresponding to the MWCNT A carbon nanotube product.

FIG. 4C depicts the carbon nanotube diameter distribution as determinedby TEM corresponding to the MWCNT B carbon nanotube product.

FIG. 4D depicts the carbon nanotube diameter distribution as determinedby TEM corresponding to the MWCNT C carbon nanotube product.

FIG. 5 is a graphical representation of electrical volume resistivityfor SMW-100 carbon nanotubes and three commercial carbon nanotubeproducts in polycarbonate.

FIG. 6A is a graphical representation of surface resistance on the frontand back of composites containing Nylon66 resin loaded with 2.5 wt %SMW-100 carbon nanotubes or loaded with 2.5 wt % of commerciallyavailable multi-wall carbon nanotubes.

FIG. 6B is a graphical representation of surface resistance on the frontand back of composites containing Nylon66 resin loaded with 3.5 wt %SMW-100 carbon nanotubes or loaded with 3.5 wt % of commerciallyavailable multi-wall carbon nanotubes.

FIG. 7 depicts surface resistivity of thin film comprising differentforms of carbon nanotubes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed disclosure of the current invention will describethe catalyst precursor, methods of preparing the catalyst precursor andconversion thereof to the desired catalyst. Additionally, the presentinvention provides methods of producing batches of desired multi-wallcarbon nanotubes on the catalyst wherein the carbon nanotube product hasa narrow range distribution of walls and external diameters. As usedherein, “carbon content” refers the percentage of the final product(carbon nanotube+catalyst) that is carbon-based. So if 250 g of thefinal product is carbon and the final product is a total of 500 g, thencarbon content is 50% or 50.0 (as used in FIG. 1). As used herein,“carbon yield” refers to the amount of carbon product produced relativeto the amount of catalyst used in the reaction. It is defined by thefollowing equation: (amount of carbon in final product (g)/amount ofcatalyst (g))×100. For example, a reaction that yields 250 g carbonproduct where 250 g of catalyst was used in the reaction would have acarbon yield of 100% ((250 g/250 g)×100=100%). As used herein,(including FIGS. 2A-2D; 2A-4D), “frequency” refers to the number ofcarbon nanotubes in a sample having a specified diameter (x axis). Forexample, in FIG. 2A, there are approximately 20 carbon nanotubes thathave a diameter of about 6 nm.

1. The Catalyst Precursor and Catalyst

The catalyst precursor of the present invention has a surface phase ofmixed metal oxides supported on a particle of alumina and magnesiumaluminate. A mixed metal oxide is an oxide having two or more metalcomponents. Additionally, the support of alumina/magnesium aluminatecarries a surface treatment of magnesium oxide. The magnesium oxidecarried by the alumina/magnesium aluminate particle is not necessarilyan encompassing layer. The atomic ratio of MgO to Al₂O₃ is between about0.02 and 0.04. Stated in other terms, for a ratio of 0.02:1, for everyatom of MgO there are 50 atoms of Al₂O₃ while at a ratio of 0.04:1, forevery atom of MgO there are 25 atoms of Al₂O₃. As noted below, a portionof the MgO used in these calculations will be converted to MgAl₂O₄.

The preferred surface phase of mixed metal oxides includes but is notlimited to, one or more of the following: CoFe₂O₄, CoMoO₄, Co_(x)MoO₄,Co_(x)Fe_(y)MoO₄, Fe₂(MoO₄)₃. Typically, the metal oxides provide thefollowing percent by weight concentrations of metals on the catalystprecursor: Co from about 0.5% to about 2.0%; Mo from about 0.3% to about2.0%; and, Fe from about 0% to about 3.0%. Thus, for Co_(x)Fe_(y)MoO₄, xmay range from about 1.6 to 6.5 and y may range from 0.1 to 10.5. Morepreferably, x will be about 3.3 and y will range from 2.6 to 6.3. In anyevent, a sufficient amount of the metal oxides are present on thecatalyst precursor such that the resulting catalyst comprises thefollowing percent by weights of the metal component: Co from about 0.5%to about 2.0%; Mo from about 0.3% to about 2.0%; and, Fe from about 0%to about 3.0%. In the resulting catalyst, the iron may be present as areduced metal or a carbide (Fe₃C), while the molybdenum is present as acarbide (Mo₂C).

Preferably, the percent by weight of each metal component based on theweight of the catalyst precursor composition is: Co, from about 0.75% toabout 1.5%; Mo, from about 0.5% to about 1.0%; and, Fe, from about 0.5%to about 2.0%. Accordingly, the active metal components are present inthe following atomic ratios, wherein Mo is constant: the ratio of Co toMo ranges from about 1.6 to about 6.5, more preferably from about 2.44to about 4.88 and most preferred about 3.3; and the ratio of Fe to Mofrom about 0 to about 10.5, more preferably about 1.75 to about 6.98 andmost preferred about 2.62 to about 6.28.

The presence of Mg ions on the catalyst support reduces the number ofstrong acid sites on the surface of the Alumina support. By reducing thenumber of strong acid sites on the surface of the catalyst support, useof the improved catalyst will produce primarily carbon nanotubes andsignificantly less amorphous carbon or other carbon products. Asdiscussed below, catalytic reactions using the improved catalyst produceat least 90% and preferably greater than 98% carbon nanotubes as theresulting carbon product.

The catalyst precursor of the present invention preferably has aparticle size between about 20 μm and about 500 μm. Preferably, theparticle size is between about 20 μm and 250 μm. More preferably, thecatalyst precursor has a particle size between about 20 μm and about 150μm. In the currently preferred method discuss below, the particles rangein size from about 70 μm to about 150 μm.

The catalyst precursor described above is converted to catalystparticles by reduction of the metal oxides to the respective metals andmetal carbides, i.e. Fe°, Fe₃C, Co° and Mo₂C. The catalyst particleshave the same atomic ratios of metal present as the catalyst precursor.The resulting nano-sized deposits of metallic cobalt and metallic ironwill determine the interior diameter of the multi-wall nanotubesproduced on the catalyst particles. Additionally, the presence of theMo₂C, disperses or spaces the metallic cobalt, thereby precludingsintering of the cobalt and providing the desired cobalt particle size.In general, the resulting metal deposits on the support will havediameters ranging from about 1.5 urn to about 3.0 nm. Preferably, theresulting metal deposits of reduced iron and reduced cobalt will havediameters ranging from about 1.5 nm to about 2.2 nm. Additionally, asnoted above, the final catalyst particles have fewer surface acid sitesthan catalyst particles utilizing only alumina as a support.

In summary, the final catalyst particles of the present invention haveparticle sizes between about 20 μm and about 500 μm. Preferably, theparticle size is between about 20 μm and 250 μm. More preferably, thecatalyst precursor has a particle size between about 20 μm and about 150μm. In the currently preferred method of making multi-wall nanotubesdiscussed below, the presently preferred particle size is about 70 μm toabout 150 μm. The catalyst particles comprise:

-   -   a. gamma alumina (γ-Al₂O₃) between about 91.0% and 97.6% by        weight, preferably between about 94.8% and about 97.3% by        weight;    -   b. Mg (in the form of MgO and MgAl₂O₄) between about 0.5% and        about 3.3% by weight, preferably between 0.5% and 1.0%;    -   c. reduced Co from about 0.5% to about 2.0% by weight,        preferably from about 0.75% to about 1.5% by weight;    -   d. Mo, in the form of Mo₂C from about 0.3% to about 2.0% by        weight, preferably from about 0.5% to about 1.0% by weight; and,    -   e. Fe, in the form of reduced iron and iron carbide (Fe°, Fe₃C)        from about 0% to about 3.0% by weight, preferably from about        0.5% to about 2.0% by weight.        Typically, less than 2.0% by weight of the catalyst particle        will be metal carbides. The atomic ratios of the reduced metals        for catalytic production of multi-wall carbon nanotubes will not        vary substantially from the catalyst precursor as the metal        carbides are not produced in substantial quantities.        2. Method of Preparing the Catalyst Precursor Particles and        Catalyst Particles

The current invention provides methods for manufacturing a catalystprecursor and a catalyst suitable for the catalytic formation ofmulti-wall carbon nanotubes. In particular, the catalyst of the currentinvention enables the production of batches of multi-wall carbonnanotubes having a narrow distribution range of walls and diameters.

In a preferred embodiment, the method initially involves preparing asolution of mixed metallic compounds comprising two or more of thefollowing: a cobalt compound selected from the group consisting ofcobalt acetate, cobalt nitrate; an iron compound selected from the groupconsisting of iron acetate, iron nitrate; a molybdenum compound selectedfrom the group consisting of ammonium heptamolybdate and ammoniumdimolybdate; and magnesium nitrate. The preferred solution comprisescobalt acetate, iron nitrate, ammonium heptamolybdate and magnesiumnitrate in water.

Regardless of the cobalt compound chosen, the solution contains aconcentration of cobalt ion between about 20 g/L and about 50 g/L; aconcentration of molybdenum ion between about 10.5 g/L and about 70.3g/L; a concentration of iron ion between about 35 g/L and about 105 g/L;and, a concentration of Mg ion between about 6.7 g/L to about 27.0 g/L.The preferred solution contains between about 26.7 g/L and about 40.0g/L cobalt ion; between about 17.6 g/L and about 35.2 g/L molybdenumion; between about 52.7 g/L and about 70.1 g/L iron ion; and, betweenabout 6.7 g/L and about 13.5 g/L magnesium ion. Most preferred is asolution of about 33.4 g/L cobalt ion; of about 17.6 g/L molybdenum ion;of about 63.1 g/L iron ion; and, about 6.7 g/L magnesium ion. Properselection of the metal ion concentration will enhance the formation ofthe desired mixed metal oxides. Thus, it is desirable to provide theproper stoichiometric ratios of the metals in solution to achieve thisresult.

The above referenced metal ions are then reacted with aluminum hydroxideto yield a mixture of metal hydroxides and other ionic compoundsincluding, but not limited to, the following hydroxides where thestoichiometric ratios may be varied from that shown: Mg(OH)₂, Fe(OH)₃,Co(OH)₂, CoMoO₄.nH₂O, Fe₂(MoO₄)₃.nH₂O. Typically the foregoing reactiontakes place at room temperature over a period of about two to fourhours. The reaction products have a paste like consistency whichpromotes agglomeration of the particles. Preferably, the paste has amoisture content of about 20% to about 40% water by weight. Morepreferably, the paste contains from about 25% to about 30% water byweight.

If necessary for agglomeration of the particles, the paste-like productis manipulated to yield agglomerated particles having particle sizesranging from about 100 μm to about 1400 μm. Typically, the particleswill agglomerate during the reaction. Preferably, the agglomeratedparticles are between about 100 μm to about 500 μm. In the preferredprocess, the agglomerated particles are mixed in a machine which kneadsor mixes the paste for about 20 to about 50 minutes. Following thekneading, the product is allowed to age for about 2 to 3 additionalhours. The total time period will depend upon the batch size. Forbatches of about 200 to about 2000 grams, the preferred kneading periodwill be about 30 minutes. Larger batches will require longer mixingtimes. Following agglomeration, the particles are dried and sieved toisolate particles less than 1400 μm. Preferably, the sieving stepprovides particles in the range of about 100 μm to about 500 μm.

The agglomerated particles are dried to a moisture content of about 10%to about 20% water by weight. Preferably, the dried particles have lessthan 15% water by weight. The drying step preferably takes place at atemperature between about 30° C. and 50° C.

Following drying and sieving, the particles are calcined under a flowinggas at a temperature between about 400° C. to about 600° C. for a periodof about 3 hours to about 8 hours. More preferably, the calcining steptakes place at a temperature between about 400° C. and 500° C. for aperiod of about 3.5 hours to about 4.5 hours. Most preferably, thecalcining step occurs at a temperature of about 440° C. to about 460°for a period of about 3.5 hours to about 4.5 hours. Preferably, thecalcining gas is selected from air, nitrogen, helium and mixturesthereof. Typically, the preferred calcining gas is a gas that is inertunder the calcining conditions. The drying and calcining steps reducethe agglomerated particles to a particle size between about 20 μm and500 μm. Alternatively, prior to calcining the particles are sieved andif necessary ground such that calcining will produce particles between20 μm and 250 μm. Preferably, calcining produces particles between about20 μm and 200 μm. More preferred are particles between about 20 μm and150 μm. In the preferred method discussed below, the preferred particlesrange in size from about 70 μm to about 150 μm. The resulting particlesare essentially free of water moisture, i.e. no greater than 3% moistureby weight.

Calcining of the particles converts the metal hydroxides to therespective oxides. For example, calcining of iron hydroxide withmolybdate yields iron-molybdate (Fe₂(MoO₄)₃). Likewise, calcining cobalthydroxide with molybdate yields cobalt-molybdate (CoMoO₄). Further,during the calcining process Fe(OH)₃ and Co(OH)₂ combine to yieldCoFe₂O₄. Finally, during calcining Mg(OH)₂ yields MgO and the aluminumhydroxide (Al(OH)₃) converts to gamma alumina, i.e. γ-Al₂O₃. During thecalcining process, the oxidation of Mg(OH)₂ also precludes the formationof strong acid sites on the surface of the γ-Al₂O₃. The resultingsurface configuration is believed to be a mixed oxide similar toMg—Al—O. In any event, the surface acidity of the γ-Al₂O₃ carrying theMgO is significantly lower than the surface acidity of γ-Al₂O₃ whencalcined without Mg(OH)₂ present.

Further, during the calcining process, in addition to forming therespective oxides of magnesium and aluminum, a portion of the Mg⁺² ionsadjacent to the aluminum hydroxide produces a parallel reaction. In thisreaction, the solubility of the magnesium ion in the alumina allows themagnesium to displace a portion of the aluminum oxide tetrahedralstructure near the surface of the particle thereby producing magnesiumaluminate (MgAl₂O₄), a compound with a spinel like structure. Theformation of magnesium aluminate is favored over the formation ofCoAl₂O₄ and FeAlO₃. Thus, the favored reaction preserves the catalyticmetals for reduction and conversion to catalyst sites on the surface ofthe resulting support particle. In particular, the reduced cobalt takesthe form of nanoparticle size domains on the surface of the resultingsupport, the iron becomes reduced iron and iron carbide and themolybdenum becomes molybdenum carbide. The iron carbide and reduced irondisperse the cobalt on the surface of the catalyst support therebycontrolling the inner diameter of the resulting nanotubes.

The resulting catalyst support has a configuration wherein magnesiumaluminate is incorporated into the crystalline structure of the γ-Al₂O₃primarily in the outer layer of the particle. Additionally, the surfaceof the gamma alumina carries MgO. Without being limited by theory, theMgO on the surface is likely a mixed oxide with the alumina of theparticle, i.e. a mixed oxide of Mg—Al—O. This configuration results fromthe reaction of the magnesium ions with the alumina during the calciningprocess. Finally, the preferred catalyst support is preferably free ofCoAl₂O₄ and FeAlO₃. If FeAlO₃ is present then preferably, the catalystsupport comprises less than 0.5 percent by weight FeAlO₃. If CoAl₂O₄ ispresent then preferably, the catalyst support comprises less than 0.5percent by weight CoAl₂O₄.

The presence of magnesium on the surface of the catalyst supportparticle reduces the surface acidity of the catalyst precursor supportparticle and the resulting catalyst support particle. By reducing thenumber acid sites on the surface of the support particle, the method ofthe current invention improves the production of carbon nanotubes andreduces the formation of other carbon types during the subsequentproduction of multi-wall carbon nanotubes. Additionally, by blocking theformation of CoAl₂O₄ and FeAlO₃ the presence of the magnesium ionprecludes the loss of catalytic metals.

Following calcining and particle size reduction, the resulting catalystprecursor particles have a catalyst support of γ-Al₂O₃/MgAl₂O₄ with asurface treatment of MgO. Additionally, the surface of the catalystsupport carries a mixed phase of the referenced metal oxides. As notedabove the preferred mixed metal oxides include but are not necessarilylimited to: CoFe₂O₄, CoMoO₄, Co_(x)MoO₄, Fe₂(MoO₄)₃, Co_(x)Fe_(y)MoO₄with Co_(x)Fe_(y)MoO₄ being the most preferred.

The resulting catalyst precursor is placed within a reaction chamber.Preferably, the reaction chamber is designed to produce a fluidized bedof catalyst particles when a flowing gas passes through the chamber andthe particles located therein. To finally convert the catalyst precursorto a catalyst, the precursor must be heated and reacted with a carboncontaining gas. In the following method for producing multi-wallnanotubes, the preferred gaseous carbon compound is ethylene. Theconversion of the catalyst precursor to catalyst takes place at atemperature between about 600° C. and 700° C. during the first tenminutes of contact with the gaseous carbon compound. During this timeperiod, the metal oxides are reduced to the respective metals and metalcarbide discussed above. Additionally, the formation of the Fe₃C andMo₂C preclude sintering and agglomeration of the reduced cobalt and ironon the surface of the support. Thus, the resulting nanoparticles ofreduced cobalt preferably have diameters between about 1.5 nm to about3.5 nm. More preferably, the reduced cobalt metal particles on thesurface of the catalyst support have diameters between about 1.5 nm and2.2 nm. The reduced iron particles will have similar sizes, i.e. fromabout 1.5 nm to about 3.5 urn preferably between about between about 1.5nm and 2.2 nm.

The resulting catalyst comprises a support of γ-Al₂O₃/MgAl₂O₄ with asurface treatment of MgO and nano size particles of Fe₃C and Mo₂C on thesurface of the support. The reduced metallic cobalt may be carried bythe γ-Al₂O₃/MgAl₂O₄ and may also be found on the molybdenum carbide(Mo₂C) and iron carbide (Fe₃C). Additionally, reduced iron may becarried by the γ-Al₂O₃/MgAl₂O₄ and may also be found on the molybdenumcarbide (Mo₂C) and iron carbide (Fe₃C).

As discussed above the resulting catalyst particles have particle sizesbetween about 20 μm and about 500 μm. Preferably, the particle size isbetween about 20 μm and 250 μm. More preferably, the catalyst has aparticle size between about 20 μm and about 150 μm. In the currentlypreferred method of making multi-wall nanotubes, the presently preferredparticle size is about 70 μm to about 150 μm.

The catalyst particles comprise: gamma alumina (γ-Al₂O₃) between about91.0% and 97.6% by weight, preferably between about 94.8% and about97.3% by weight; Mg (in the form of MgO and MgAl₂O₄) between about 0.5%and about 3.3% by weight, preferably between 0.5% and 1.0%; reduced Cofrom about 0.5% to about 2.0% by weight, preferably from about 0.75% toabout 1.5% by weight; Mo, in the form of Mo₂C from about 0.3% to about2.0% by weight, preferably from about 0.5% to about 1.0% by weight; and,Fe, in the form of reduced iron and iron carbide (Fe°, Fe₃C) from about0% to about 3.0% by weight, preferably from about 0.5% to about 2.0% byweight. Typically, less than 2.0% by weight of the catalyst particlewill be metal carbides. The atomic ratios of the reduced metals forcatalytic production of multi-wall carbon nanotubes will not varysubstantially from the catalyst precursor as the metal carbides are notproduced in substantial quantities.

In an alternative method for preparing the catalyst precursor, themagnesium nitrate has been omitted from the initial solution. In thismethod, magnesium hydroxide powder is combined with the aluminumhydroxide powder and reacted with the solution of metallic compoundscomprising a cobalt compound selected from the group consisting ofcobalt acetate, cobalt nitrate, an iron compound selected from the groupconsisting of iron acetate, iron nitrate, a molybdenum compound selectedfrom the group consisting of ammonium heptamolybdate and ammoniumdimolybdate and mixtures thereof. The preferred solution comprisescobalt acetate, iron nitrate, ammonium heptamolybdate and magnesiumnitrate in water.

Regardless of the cobalt compound chosen, the solution contains aconcentration of cobalt between about 20 g/L and about 50 g/L; aconcentration of molybdenum ion between about 10.5 g/L and about 70.3g/L; a concentration of iron ion between about 35 g/L and about 105 g/L;and, a concentration of Mg ion between about 6.7 g/L to about 27.0 g/L.The preferred solution contains between about 26.7 g/L and about 40.0g/L cobalt ion; between about 17.6 g/L and about 35.2 g/L molybdenumion; between about 52.7 g/L and about 70.1 g/L iron ion; and, betweenabout 6.7 g/L and about 13.5 g/L magnesium ion. Most preferred is asolution of about 33.4 g/L cobalt ion; of about 17.6 g/L molybdenum ion;and about 63.1 g/L iron ion.

The solution of metal ions is subsequently reacted with an excess ofaluminum hydroxide powder having particles ranging in size from about 20μm to about 150 μm and magnesium hydroxide powder having particlesranging in size from about 20 μm to about 150 μm. Following thisreaction, the preparation of the catalyst precursor and the subsequentcatalyst is identical to the process described above.

Manufacture of Multi-Wall Carbon Nanotube Batches having NarrowDistribution Ranges of Walls and Diameters

The following discussion concerning the catalytic production ofmulti-wall carbon nanotubes is essentially a continuation of thediscussion above concerning the preparation of the catalyst precursorand the catalyst. Following placement of the calcined catalystprecursors in the reactor chamber, the particles are fluidized andconverted to catalyst particles. As noted above, the catalyst may haveparticle sizes between about 20 μm and about 500 μm. Preferably, theparticle size is between about 20 μm and 250 μm. More preferably, thecatalyst precursor has a particle size between about 20 μm and about 150μm. In the currently preferred method of making multi-wall nanotubes,the presently preferred particle size is about 70 μm to about 150 μm.Thus, the particles are well suited for use in a fluidized bed reactor.

Following placement of the catalyst precursor particles in the reactionchamber, a flowing stream of nitrogen gas passes through the reactionchamber thereby fluidizing the bed of particles. The nitrogen gas isheated to a temperature sufficient to raise the temperature within thefluidized bed to a range of about 600° C. to about 700° C.Alternatively, the reaction chamber may be located in a furnace or othersuitable heating device. When located within a furnace, the reactionchamber will typically be heated by both the furnace and the gas. Morepreferably, the fluidized bed is pre-heated to a temperature betweenabout 600° C. to about 650° C. Most preferably, the fluidized bed ispre-heated to about 610° C. to about 630° C. One skilled in the art willrecognize that other non-reactive gases such as argon or helium may besubstituted for nitrogen. The primary requirement for the pre-heatingstep is fluidization and heating of the fluidized bed to the desiredtemperature without undesirable side reactions.

Upon stabilization of the temperature within the fluidized bed, the gasflow to the bed is switched from nitrogen to a reactive gas. Thereactive gas is a non-reactive carrier gas with a carbon containing gas.The preferred carrier gas is nitrogen and the preferred carboncontaining gas is ethylene; however, other carrier gases such as argonor helium will perform equally well. The preferred blend of ethylene innitrogen by volume is between about 10% and 80% by volume. Morepreferably, the reactive gas contains from about 20% to about 50% byvolume ethylene in nitrogen. Most preferred is a reactive gas containingfrom about 20% to about 40% by volume ethylene in nitrogen.

The flow rate of the ethylene containing gas is not dependent upon thesize of the reaction chamber. Rather, the volume of gas passing throughthe reaction chamber depends upon the grams of catalyst precursor withinthe reaction chamber. The flow rate will be from about 70 L/min per kgof catalyst precursor to about 150 L/min per kg of catalyst precursor.More preferably, the flow rate will range between about 90 L/min per kgof catalyst precursor to about 120 L/min per kg of catalyst precursor.

The initial reaction of the ethylene containing gas with the catalyticparticles reduces the metal oxides to their respective metals (Co° andFe°) and metal carbides (Mo₂C and Fe₃C). This reduction step generallyoccurs over the first five minutes of the reaction process. Preferably,the reaction temperature is 600° C. to 750° C. More preferably, thereaction temperature is between 610° C. and 650° C. Most preferably, thereaction temperature is 610° C. Additionally, during the first tenminutes of the reaction process, the ongoing reaction of ethylene withthe catalyst precursor and subsequent catalyst particles is anexothermic reaction. Thus, the preferred method maintains thetemperature of the fluidized bed below 670° C. Temperature maintenancemay be achieved by lowering the temperature of the gas entering thereaction chamber. If a furnace is used, then the temperature of thefurnace may also be reduced. Preferably, the temperature is maintainedbelow 650° C. as higher temperature will lead to an increased productionof amorphous carbon. As the metal oxides are reduced, the ethylene gascontacts the resulting catalytic particles and begins to grow multi-wallcarbon nanotubes. Following the reduction of metal oxides to catalyticparticles, the reaction process continues for about 10 to about 40minutes. More preferably, the reaction process following the reductionof metal oxides continues for about 15 to 25 minutes.

The resulting carbon product carried by the now spent catalyst particlesis 98% free of amorphous carbon and other carbon forms. Thus, 98% of thecarbon product is multi-wall carbon nanotubes. Further, the resultingmulti-wall carbon nanotubes primarily have from 3 to 8 walls. Morepreferably, the resulting nanotubes carried by the spent catalystparticles primarily have from 3 to 6 walls and external diametersbetween about 4.0 nm to about 7.0 nm. Preferably, at least 60% of theresulting multi-wall carbon nanotubes have three to six walls andexternal diameters between about 4.0 nm and about 7.0 nm. Morepreferable, the method of the current invention yields multi-wall carbonnanotubes wherein at least 75% of the resulting multi-wall carbonnanotubes having the desired narrow distribution range of 3 to 6 wallsand diameters between about 4.0 nm and 7.0 nm. More preferably, at least85% of the resulting nanotubes carried by the spent catalyst have threeto six walls and external diameters between about 4.0 nm and about 7.0nm. Most preferably, with continuously maintained fluidization of thecatalyst particles, the present invention will provide spent catalystcarrying multi-wall carbon nanotubes wherein 90% of the resultingmulti-wall nanotubes will have 3 to 6 walls and diameters between about4.0 nm and about 7.0 nm.

The following examples and test data do not limit the nature of thecurrent invention. Rather, this information will enhance theunderstanding of the current invention.

Example 1 Objective

This example demonstrates the effect of various catalyst metalcompositions on carbon yield and carbon nanotube diameter.

Methods

A variety of catalyst precursors were prepared to demonstrate theimportance of the catalytic metals on the resulting multi-wall product.The table in FIG. 1 identifies the nanotube products produced for thiscomparison. For these examples, 600 grams of catalyst precursor preparedas discussed above, having particles sizes of 150 to 300 microns, wereplaced in a fluidized bed reactor. As discussed above, the method of thecurrent invention converts the catalyst precursor to catalyst andsubsequently grows multi-wall carbon nanotubes on the resultingcatalyst. For each of the examples provided in FIG. 1, the finalcatalysts were reacted at 610° C. with 40% ethylene in nitrogen at a gasflow rate of 60 L/min (gas flow/mass of catalyst ratio of 100 L/min perKg catalyst) for 20 minutes.

Results

As depicted in FIG. 1, the catalytic metal composition significantlyimpacts the resulting multi-wall nanotube product. For example, runsPXE2-282, PXE2-285, PXE2-292 and PXE2-293, provide data regardingmulti-wall carbon nanotubes prepared with catalyst precursors having Co,Mo and from about 0.75 percent weight of iron to about 1.9 percentweight of iron. The resulting batch of nanotubes have a high yield ofcarbon nanotubes with a median external diameter from about 6.72 nm toabout 8.24 nm and a mode external diameter from about 4.97 nm to about 6nm. Between 75% and 85% of these carbon nanotubes have externaldiameters of less than 10 nm. Specifically, PXE2-282 represents a batchof multi-wall nanotubes having a mode diameter of 6.0 nm, the mediandiameter for the batch is 8.24 nm and 73% of the batch had diametersless than 10 nm. Similarly, PXE2-285 represents a batch of multi-wallnanotubes having a mode diameter of 5.38 nm, the median diameter for thebatch is 6.72 nm and 85% of the batch had diameters less than 10 nm. Thevalues for PXE2-292 and PXE2-293 can be easily determined from FIG. 1.As known to those skilled in the art, the term “mode” when used in thismanner represents the value that occurs the most frequently in a dataset. Thus, for PXE2-285, the most common diameter for nanotubes withinthe batch is 6.72 nm.

These results demonstrate that catalyst precursor compositionscomprising Co from about 0.75 to about 1 percent weight of total metalsof catalyst precursor, Fe from about 0.75 to about 1.9 percent weight oftotal metals of catalyst precursor, and Mo from about 0.4 to 0.5 percentweight of total metals of catalyst precursor, result in high percentageyields of small diameter carbon nanotubes.

In contrast, catalyst precursor particles lacking iron result in asignificant reduction in carbon yield. For example, run PXE2-288demonstrates a 57% loss in carbon yield when iron is removed from theprecursor catalyst formulation. Interestingly, the resulting productcomprises carbon nanotubes with a median external diameter of 6.98 and amode external diameter of 4.68. This suggests that iron is notresponsible for the small diameter of the resulting carbon nanotubes.However, the results seem to suggest that molybdenum plays a role inlimiting carbon nanotube diameter. For example, run PXE2-284, producedcarbon nanotubes having a median and mode external diameter of 9.63 nmand 11.06 nm, respectively. Additionally, only 54% of the resultingcarbon nanotubes had an external diameter less than 10 nm compared to85% in run PXE2-285, where Mo was used in the precursor composition.FIGS. 2A-2D further illustrates the effect of removing either Fe or Mofrom the catalyst precursor on carbon nanotube diameter distribution.Taken together, these results demonstrate that iron acts to maintaincarbon yield while molybdenum promotes production of a smaller diametercarbon nanotube.

Example 2 Objective

With reference to FIG. 3, this example demonstrates the effect ofreaction temperature and gas composition on carbon yield and carbonnanotube diameter.

Methods

Catalyst compositions having the formulations of PXE2-282 and PXE2-285in FIG. 1 were used in this test as a reference. To determine the impactof reaction temperature on the resulting nanotube product, reactionswere carried out at temperatures between 610-675° C. Further, thesetests determined the impact on the resulting nanotube product due tochanges in ethylene concentration in the gas feed for variations ofethylene concentration between 30-40%.

Results

Increasing the reaction temperature and/or lowering the gas compositionfrom 40% to 30% ethylene decreases the carbon yield and increases thediameter of the carbon nanotubes. Thus, in order to maximize carbonyield and to produce small diameter carbon nanotubes, the catalyticreaction should occur at about 610° C. with a reactive gas mixturecontaining 40% ethylene.

Example 3 Objective

This example compares the electrical conductivity of compositescontaining primarily small diameter multi-wall carbon nanotubes havingbetween 3-6 walls (diameters of 4-8 nm) to composites comprising largerdiameter carbon nanotubes. This example and the following examplesutilize the material prepared according to the current invention andidentified as PXE2-282 in FIG. 1 (referred to as SMW-100).

Methods

Carbon nanotubes produced by the methods and catalyst compositions ofthe current invention (hereinafter, SMW-100 refers to multi-wall carbonnanotubes produced by the catalyst composition described for PXE2-282 inFIG. 1) were compared to various commercially available carbon nanotubeshaving diameter distributions described in Table 1 and FIGS. 4A-D. Thefollowing Table 1 provides the carbon nanotube diameter distributionsfor various commercially available multi-wall carbon nanotubes andSMW-100. For example, with regard to SMW-100 ten percent of thenanotubes have diameters smaller than 4.2 nm, 50% of the total nanotubeshave diameters smaller than 6.7 nm and 90% of the total nanotubes havediameters smaller than 12 nm.

TABLE 1 10% 50% 90% SMW-100 4.2 nm 6.7 nm 12.0 nm MWCNT A 5.5 nm 7.8 nm13.0 nm MWCNT B 7.4 nm 12.0 nm  16.5 nm MWCNT C 7.1 nm 9.9 nm 13.3 nm

Polycarbonate Makrolon 2600 PC granules were melt mixed with the carbonnanotube sources described in Table 1. Melt mixing was performed in aDSM micro-compounder (15 cm3) under the following conditions: screwspeed—200 rpm; temperature—280° C.; time—5 min) Pressed plates (60 mmdiameter×0.5 mm thickness) were prepared from extruded strands(temperature: 280° C., time: 1 min, pressure: 100 kN). Carbon nanotubesamples were characterized by TGA and TEM analysis (FIGS. 4A-D).

Resistivity was measured with a Keithley 6517A Electrometer incombination with a Keithley 8009 test fixture (for resistivity >10⁷ Ohmcm) or a strip test fixture (for resistivity <10⁷ Ohm cm). For thepurposes of this disclosure, the term percolation threshold is thatconcentration of carbon loading at which there is one, and only one,continuous conducting pathway in the material.

Results

FIG. 5 demonstrates that the SMW-100 carbon nanotube material providesthe lowest electrical percolation threshold. As depicted in FIG. 5, aCNT loading of 0.33 wt. % satisfied the requirement for electricalpercolation. As shown by FIG. 5, SMW-100 provided resistivity reading of10⁴-10² Ohm/cm for loadings ranging from 0.5-1.0 wt %. In contrast, thecomparative carbon nanotubes having diameters between 7-9 nm (MWNT A),10-11 nm (MWNT C), and 12-15 nm (MWNT B) respectively yieldedpercolation thresholds of 0.50 wt %, 0.50 wt % and 0.55-0.60 wt %.

Based on the above, results, the use of a batch of straight multi-wallcarbon nanotubes having the characteristics of SMW-100 provided by Table1 and FIG. 1 will provide higher conductivity properties at lowerloading levels than other commercially available sources of multi-wallcarbon nanotubes.

Example 4 Objective

This study compares the performance of composites based on commerciallyavailable multi-wall carbon nanotubes dispersed in Nylon 66 resin tocomposites prepared from SMW-100 carbon nanotubes dispersed in Nylon66resin.

Methods

CNT-Nylon 6,6 compounding was performed via twin screw extrusion. Theresulting composites were then injection-molded into standard ASTM testbars and plaque (4 in by 4 in by 3.2 mm). Conductivity measurements werethen performed on the injection-molded plaques using a standard ProStatresistance meter as per ASTM D-257 for Volume and Surface resistance.Surface resistance was determined using PRF-912B probe at 25predetermined locations on each surface of the injection moldedplaques—i.e. 25 points on the front surface and 25 points on the backsurface of the plaques. This rigorous testing is designed to bring outany minor variations in electrical performance due to non-uniformity inmaterial and/or processing. The front surface of the plaques correspondsto where the ejector pins are located. The back surface of the plaquescorresponds to the fixed part of the tool (closer to Nozzle). Volumeresistances of the plaques were tested using a PRF-911 concentric ringat five locations per sample and averaged for both the front and back ofthe plaques.

Results

The surface resistance data is depicted in FIGS. 6A and 6B. SMW-100composites exhibited lower and more uniform electrical resistanceproperties after molding compared to the commercially availablemulti-wall carbon nanotube (MWCNT). The surface resistance, of the MWCNTand SMW-100 filled samples are fairly uniform and consistent with verygood agreement on the front and back surfaces of the plaques.

Furthermore, Nylon 6,6 based composite with SMW-100 showed higherconductivity values than Nylon 6,6 based composite with commerciallyavailable grades of MWCNT. The SMW-100 composites also showed moreuniform resistance values, with a narrower range of standard deviationbetween the tested points and between the front and back surfaces of theplaques. As reflected by FIGS. 6A and 6B, composites prepared from theinventive carbon nanotube material, i.e. a batch of nanotubes having anarrow distribution of diameters and number of walls, have improvedconductivity when compared to currently available materials.

Example 5 Objective

This example compares the surface resistivity of thin films containingrespectively: SMW-100; single-wall carbon nanotubes (SWNT); double-wallcarbon nanotubes (DWNT); and, commercially available multi-wall carbonnanotubes (MWCNT B—from Example 3).

Methods

Carbon nanotube-based thin films having different degrees oftransparencies (80-95% transmittance) were prepared using solutionscontaining 1 g carbon nanotube/liter in 1% Triton-X100 surfactant. Thesolutions were then sonicated and centrifuged. The various carbonnanotube inks were deposited on a PET 505 substrate employing the rodcoating technique.

Results

As depicted in FIG. 7, films having 80-90% transparency prepared withSWNTs demonstrate higher electrical conductivity than the other type ofcarbon nanotube materials in thin film. However, films prepared usingthe novel batch material of the current invention, i.e. the SMW-100, hadbetter conductivity performance than films incorporating conventionalDWNT and MWNT.

Other embodiments of the current invention will be apparent to thoseskilled in the art from a consideration of this specification orpractice of the invention disclosed herein. Thus, the foregoingspecification is considered merely exemplary of the current inventionwith the true scope and spirit of the invention being defined by thefollowing claims.

We claim:
 1. A catalyst precursor composition comprising: a supportcomprising alumina and magnesium aluminate; and with magnesium oxide andmixed metal oxides on a surface of the support, wherein the mixed metaloxides are selected from the group consisting of Co_(x)Fe_(y)MoO₄,Fe₂(MoO₄)₃ and blends thereof and wherein the atomic ratio of magnesiumoxide to alumina is between about 0.02 to about 0.04, wherein x and yfor the mixed metal oxides represented by Co_(x)Fe_(y)MoO₄ is from about1.6 to 6.5, and 0 to 10.5, respectively.
 2. The catalyst precursorcomposition of claim 1, wherein x is from about 2.44 to 4.88 and y isfrom about 1.75 to 6.98.
 3. The catalyst precursor composition of claim1, wherein the mixed metal oxide is selected from the group consistingof Co_(3.3)Fe_(2.62)MoO₄ and blends thereof.
 4. The catalyst precursorcomposition of claim 1, wherein the mixed metal oxide is selected fromthe group consisting of Co_(3.3)FeyMoO₄ and blends thereof, where y mayrange from 2.6 to 6.3.
 5. The catalyst precursor composition of claim 1wherein the alumina is gamma-alumina.